Optical fiber cables carrying optical fiber transmission lines have heretofore been installed by methods which are used to install metallic conductor cables. Such methods may involve the step of pulling the cable with a pulling rope through a previously installed cable conduit or duct.
Unlike the metallic conductors of an insulated metallic conductor cable, optical fibers are easily damaged by tensile stress. Such stress may, for example, propagate micro-cracks, leading to fiber breakage over a period of time. In order to avoid undue stresses in the fiber and consequent impairment of transmission, optical fiber cables may be reinforced, in some instances by providing a central strength member, comprising one or more steel wires, about which the optical fibers are disposed. Strength members instead may be disposed as elements of the sheath system. The strength members take up tensile stresses and thereby allow the cable to undergo the rigors accompanying installation of the cable.
Unfortunately, the strength members may provide insufficient protection against local stresses caused by pulling a cable through a duct having a tortuous configuration, for example. The problem of having to pull a cable into a duct to add transmission capacity may be overcome by installing at the outset sufficient optical fiber cables which have sufficiently large numbers of optical fibers to satisfy forseeable future traffic demands. The installation of optical fiber cables which initially include relatively large numbers of optical fibers may be in order, notwithstanding the need for only a small fraction of the installed fibers to provide traffic carrying capacity that is ample for initial requirements.
The initial installation of large diameter optical fiber cables which include relatively high numbers of optical fibers is undesirable for a variety of reasons. Firstly, there are problems of a technical nature inherent in such cables, such as, for example, the difficulty of forming connections and of achieving required high strength-to-weight ratios. Secondly, there are clear economic drawbacks in committing capital resources to install initially unused optical fiber capacity. This is particularly valid in view of recent advances in optical fiber technology which lead one to expect continued substantial reductions in the price and improvement in the quality of optical fiber. Thirdly, there is the serious risk of damaging in a single incident very large numbers of expensive optical fibers. Finally, the routing of optical fiber transmission lines becomes difficult as flexibility decreases with increasing diameter.
As an alternative to the initial installation of relatively large diameter optical fiber cables, improved methods of installation of optical fiber for additional capacity have been sought. Methods of installing optical fiber cables in ducts are known in the art. A method of installing optical fibers with pulling ropes is described by Herman S L Hu and Ronald T. Miyahara in an article entitled "Subducts: The Answer to Honolulu's Growing Pains" which appeared at pages 22-35 of the Apr. 7, 1980 issue of Telephony. The installation method described in the foregoing publication includes the step of rodding out a section of an existing 100 mm duct. Thereafter 25 mm polyethylene tubes are inserted into the duct by using pulling ropes. The polyethylene tubes form subducts into each of which an optical fiber cable may be pulled with the aid of a nylon pull cord which has previously been inserted into the subduct. The pull cord may be inserted by means of a parachute which is attached to a leading end of the cord and which is pushed through the subduct with compressed air or pulled therethrough by vacuum.
The just-described method deals with some of the problems discussed hereinbefore. It enables fiber capacity to be increased as demand increases and separates optical fiber cables being installed from those cables already in a duct, thereby reducing the likelihood of wedging, and hence overstressing, of optical fiber cable.
In another method, optical fiber cable is installed in a fluid transmission conduit by inserting a pig or the like into the conduit at one end thereof and connected to one end of an optical fiber cable or pilot wire which is inserted into the interior of the conduit through a fluid seal. The pig, which is connected to one end of the cable or pilot wire for towing purposes, is pumped through a predetermined portion of the conduit, preferably by a liquid such as water, to provide a relatively smooth traversal. The pig is retrieved at the receiving end of the predetermined conduit portion where the cable is disconnected from the pig and terminated by a suitable connector exterior to the conduit or connected to a further section of cable for extension through an additional portion or portions of conduit.
An extensive network of conduits which are capable of having fluid flow therethrough and which already are in existence in the United States and other countries include those which have been used for the transportation of crude oil, refined petroleum liquids and other hydrocarbon fluids. This is particularly advantageous with respect to the use of both active and inactive pipelines for receiving optical fiber cables and the like. Fluid transmission pipelines are particularly attractive for use in conjunction with stringing optical fiber cables because the pipeline normally extends along a direct route between major points of sending and receiving communication signals and provides mechanical protection for the cables themselves. Further, the extensive network of pipelines already in existence minimizes the effort required in acquiring right-of-way for installation of the cable.
In still another prior art method of installing optical fiber cable, a leading end of optical fiber is inserted along a tubular pathway comprising a previously installed conduit. The cable is propelled along the pathway by fluid drag of a gaseous medium which is flowed through the conduit in the desired direction of advance.
The installation of fiber by the use of flowing air is a somewhat new and advantageous method for installing optical fiber transmission lines. A bundle of optical fibers also may be propelled along a conduit by the fluid drag of compressed air which is fed into the duct. It will be appreciated that to generate sufficient fluid drag to propel the optical fiber transmission medium, the gaseous medium has to be flowed through the conduit at a velocity which is substantially greater than the desired rate of advance. Typically, the inner diameter of a conduit which is suitable for such an installation technique is 6 mm, the outer diameter of the fiber bundle is 2 mm, its mass is 2 to 3 g/m and the pressure is approximately 10.sup.6 Pa. Suitable material for both the conduit and the fiber packaging is polyethylene.
There are disadvantages with these techniques insofar as their use to move optical fiber cable into existing conduits in the field is concerned. The larger the conduit in which the optical fiber is to become disposed, the more difficult it is to propel the optical fiber by use of pressurized air. Other prior art schemes which include, for example, the use of a pig are not altogether satisfactory. First of all, the conduit into which the optical fiber is to be moved may not be sufficiently large to accommodate a pig. Further, a pig may be immobilized in a conduit having a tortuous configuration.
What is desired are methods which overcome these drawbacks. Also, the sought-after methods and apparatus must be capable of being used to move optical fiber ribbons into a conduit. An optical fiber ribbon includes an array of optical fibers held between two tapes or held together as is shown in U.S. Pat. No. 4,900,126 which issued on Feb. 13, 1990 in the name of K. W. Jackson, G. A. Lochkovic, P. D. Patel, M. L. Pearsall, and J. R. Petisce.
What is sought-after and what seemingly is not available are methods and apparatus for moving optical fiber into existing conduits without the need for pressurized air. Further, the methods and apparatus should be such that they may be used to move ribbons, for example, into conduits which are relatively large in internal diameter and to move the fiber or fiber ribbon cables along a tortuous path.